In industry, level measuring devices are employed for monitoring a predetermined fill level of a liquid, often for overfill protection and for preventing pumps from running dry. A vibrating limit fill level switch, with which density and viscosity can also be determined, is described, for example, in DE 4419617 C2. Disclosed is a unit capable of oscillating, an electromechanical transducer unit for inciting the unit capable of oscillating to mechanical oscillation by means of electrical transmission signals and for receiving the mechanical oscillation of the unit capable of oscillating and transducing the same into an electrical receiving signal, an evaluation unit, which determines, based on the frequency of the receiving signal, whether the unit capable of oscillating is covered, and a regulating circuit, which regulates a phase difference that exists between the electrical transmission signal and electrical receiving signal to a constant value, wherein the value is selected such that the unit capable of oscillating oscillates at the resonant frequency. In the simplest case, the regulating circuit consists of an amplifier and a phase shifter, wherein the receiving signal is supplied to the amplifier and is fed back to the transmission signal via the phase shifter.
The electromechanical transducer unit is generally formed from one or more piezoelectric elements. In this, either one piezo-element embodies both transmitter and receiver, or a plurality of piezo-elements, which each only form a transmitter or receiver are arranged in a stack.
The unit capable of oscillating, the transducer unit and the regulating circuit form the components of an oscillating system, wherein the components are not fully separated electrically and mechanically so that mechanical and electrical coupling effects arise. These reveal themselves in the form of additional signals in the receiving signal which are superimposed on the measurement signal and, depending on the embodiment of the transducer unit, comprise a phase shift of 0° or 180°, relative to the transmission signal. The measurement signal here has the form of a sine wave, while the additional signals due to coupling are square wave signals.
As long as the unit capable of oscillating oscillates, undamped, at the resonant frequency, the coupling effects are negligible. However, with increasing damping, the amplitude of the actual measurement signal becomes smaller and the superimposed square wave signal gains significance. In this, the phase shift between transmission signal and receiving signal shifts from 90° in the direction of 0° or 180°, respectively. If the coupling dominates, the oscillating system preferably oscillates at a proscribed mechanical cut-off frequency at 0° or 180°, according to the phase shift between coupling and transmission signal. In dependence on the embodiment on the electronics unit, no mechanical cut-off frequency for the respective phase shift exists and the oscillating system no longer oscillates at all. The higher the damping of the oscillation, the earlier mechanical oscillation at the resonance frequency is no longer possible. It is therefore necessary to suppress the coupling as far as possible in order to increase the maximum viscosity at which the unit capable of oscillating is still able to oscillate.
In the prior art, this problem is met by a particular embodiment of the electromechanical transducer which eliminates, firstly, the dependence of the phase difference, to which the resonance frequency corresponds, on the performance of the oscillating system, and secondly, compensates the coupling effects. A solution variant for an electromechanical transducer unit, which consists of a single piezoelectric element, which is simultaneously a transmitter and receiver, is described in EP 0875740 A1. The piezoelectric element is subdivided into three responses, wherein a first and a second region are in reverse polarization with respect to a third region. The first and third regions serve as a transmitter, the second as a receiver. Through the reverse polarization, the two transmission signals are in phase opposition. Thus, the two additional signals due to coupling are, just as the transmission signals, in phase opposition and comprise equal amplitudes so that they mutually compensate each other and the coupling is eliminated. In EP 0875742 A1 a solution is described for the case where an electromechanical unit comprises at least one transmitter and at least one receiver, e.g. piezo-elements arranged in the form of a stack. The solution consists therein, that the receiving signal transmission line is connected to the transmitter transmission line via electrical impedance. This impedance is chosen so that the additional signals that arise due to mechanical and electrical coupling compensate for each other. Both variants solve the problem of coupling by reconfiguring the fork component assembly. Hence, it is not possible to retroactively reconfigure level measuring devices that have conventional transducer units.